Help 3rd Post

[beav33]

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /homepages/0/d154345321/htdocs/ohire/member.php5:2) in /homepages/0/d154345321/htdocs/ohire/member.php5 on line 6

Could somebody help me with this.
Explain what it is telling me, like header meaning the actual header or is it something else.
I am trying to start a session using the username.

This is the member.php5 code for session

<?php
require_once('this_file.php5');
session_start();

//create short variable names
$username = $_Post['username'];
$password = $_Post['password'];

if ($username && password)
// they have just tried logging in

{
try
{
login($username, $password);
//if they are in the database register the user id
$_SESSION['valid_user'] = $username;

}
catch(Exception $e)
{
//unsuccessful login
echo 'You could not be logged in.
You must be logged in to view this page';
exit;
}
}
?>
Then the html
Sorry its long and messy
Please help

[.josh]

change this:
[code]
<?php
require_once('this_file.php5');
session_start();
[/code]

to this:
[code]
<?php
session_start();
require_once('this_file.php5');
[/code]